The objective of the proposed research is to devise an in vivo model system that permits the study of the interaction of tumor viruses (SV40 and RNA tumor viruses) with the cells of an organism of various stages of differentiation under highly defined conditions. Very early mouse embryos (fertilized one cell eggs) will be infected with the oncogenic viruses by microinjection and will subsequently be allowed to develop to adult mice. If the integration of viral genetic information into the mouse chromosomes occurs at this earliest stage of development (the one cell egg), every cell of the organism should contain the viral information. Molecular hybridization, biochemical and serological techniques will be used to study the presence of viral genes, viral gene expression and virus induced oncogenesis at the different developmental stages and in the various differentiated tissues throughout the life of the animal. The relation between the state of differentiation of a cell and its response to an oncogenic challenge is of fundamental importance for our understanding of carcinogenesis.